1. Technical Field
The present invention relates to an ultrasonic transducer, an ultrasonic speaker, and a method of controlling the driving of the ultrasonic transducer, and in particular, it relates to an electrostatic ultrasonic transducer, an ultrasonic speaker, and a method of controlling the driving of the ultrasonic transducer capable of outputting sound waves in response to input signals with high fidelity.
2. Related Art
Typical ultrasonic transducers include a piezoelectric transducer and an electrostatic transducer. The piezoelectric transducer uses a piezoelectric element such as piezo as a vibrator and is a resonant transducer of the type that operates using its resonant frequency band. Thus, it has the characteristic of generating a high sound pressure efficiently but having a narrow-band frequency response. On the other hand, the electrostatic transducer is of the type that vibrates a thin electrode layer by applying electrostatic force between a fixed electrode and the electrode layer and has a wide-band frequency response.
When a modulated wave (sound wave) generated by modulating the amplitude of a high sound-pressure ultrasonic carrier wave by an audio-range signal is radiated into air, the sound speed is high at a high sound pressure and low at a low sound pressure, generating distortion in the waveform as the sound wave propagates in the air. It is known that the distortion is accumulated as the sound wave propagates in the air, attenuating the carrier wave component gradually, so that the audio-range signal component used for modulation is auto-demodulated. The phenomenon is called a parametric array. Since the auto-demodulated audio sound has high directivity because of ultrasonic transmission, a speaker based on the principle is called a parametric speaker or a superdirectivity speaker (ultrasonic speaker).
Since a superdirectivity speaker (ultrasonic speaker) needs a high sound pressure, related-art superdirectivity speakers generally use a resonant transducer (for example, refer to JP-A-2003-47085 and JP-A-2004-112212). The related-art superdirectivity speakers, however, are regarded as having a lower reproduction sound quality than loud speakers, so that they are used only for audio application such as local announcement and exhibition commentary. Thus, the resonant transducer has a narrow-band frequency response with a limited drive frequency, thus having the problems that it is difficult to improve the reproduction sound quality and to control the reproduction range. It also has the problem of being sensitive to an excessive input, tending to damage the elements, thus requiring some cautions.
On the other hand, an electrostatic transducer has the characteristic of generating a lower sound pressure per unit area than that of the resonant transducer but having a wide-band frequency response. Accordingly, it can easily improve the reproduction sound quality and control the reproduction range. Also, since it has a more flexible vibrator (film) than that of the resonant transducer, it is hardly damaged by an excessive input, thus having the advantage of not needing nervous (careful) handling as with the resonant transducer.
Thus, it is desirable to construct the superdirectivity speaker using the electrostatic transducer in view of improvement in reproduction sound quality and ease of handing.
The electrostatic transducer is roughly classified into two types: a pull type; and a push-pull type, in terms of structure. Their advantages and disadvantages are as follows.
FIGS. 11A and 11B are diagrams for explaining the concept of driving a pull electrostatic ultrasonic transducer. An alternating current signal that is superimposed on a direct-current bias outputted from a DC bias supply is applied between a diaphragm 10 formed by evaporating a conducting layer on an insulating film and a fixed electrode 20, whereby the diaphragm 10 is vibrated to output ultrasonic waves.
FIG. 11A shows the amplitude of the diaphragm 10 to which the positive (+) output of the alternating current signal superimposed on the direct current bias is applied. FIG. 11B shows the amplitude of the diaphragm 10 to which the negative (−) output of the alternating current signal superimposed on the direct current bias is applied.
In the state of FIG. 11A, the potential difference between the fixed electrode 20 and the diaphragm 10 increases, applying high electrostatic force between the fixed electrode 20 and the diaphragm 10 to draw the center of the diaphragm 10 toward the fixed electrode 20. In the state of FIG. 11B, the potential difference between the fixed electrode 20 and the diaphragm 10 decreases, decreasing the electrostatic force (attracting force) between the fixed electrode 20 and the diaphragm 10 to draw back the center of the diaphragm 10 in the direction opposite to the fixed electrode 20 by elastic restoration. Thus the diaphragm 10 vibrates in response to alternating signals, thereby generating ultrasonic waves.
Unlike the push-pull electrostatic ultrasonic transducer (to be described later), the pull electrostatic ultrasonic transducer does not need to have a through hole in the fixed electrode for sound waves to pass through, thus having the advantage of having a high aperture ratio to facilitate obtaining a high sound pressure. On the other hand, it has the disadvantage of having an output waveform with large distortion because its vibration conductive component is only the electrostatic attracting force and the elastic restoring force of the diaphragm.
FIGS. 12A to 12C are diagrams for explaining the concept of driving a push-pull electrostatic ultrasonic transducer. The push-pull electrostatic ultrasonic transducer has upper fixed electrodes 20a and lower fixed electrodes 20b opposed to the diaphragm 10. A positive (+) DC bias is applied to the diaphragm 10 from the DC bias supply, and an alternating current signal is applied between the upper fixed electrodes 20a and the lower fixed electrodes 20b. 
FIG. 12A shows the amplitude of the diaphragm 10 when the alternating signal is zero (0). The diaphragm 10 is in the neutral position (in the center of the upper fixed electrodes 20a and the lower fixed electrodes 20b). FIG. 12B shows the amplitude of the diaphragm 10 when the positive (+) voltage of the alternating current signal is applied to the upper fixed electrodes 20a and the negative (−) voltage of the alternating current signal is applied to the lower fixed electrodes 20b. The center of the diaphragm 10 is drawn toward the lower fixed electrodes 20b by the electrostatic force. (attracting force) between it and the lower fixed electrodes 20b and the electrostatic force (repulsion) between it and the upper fixed electrodes 20a. 
FIG. 12C shows the amplitude of the diaphragm 10 when the positive (+) voltage of the alternating current signal is applied to the lower fixed electrodes 20b. The center of the diaphragm 10 is drawn toward the upper fixed electrodes 20a by the electrostatic force (attracting force) between it and the upper fixed electrodes 20a and the electrostatic force (repulsion) between it and the lower fixed electrodes 20b. Thus the diaphragm 10 vibrates in response to alternating signals, thereby outputting ultrasonic waves.
The push-pull electrostatic ultrasonic transducer has the advantage of having an output waveform with small distortion because both the electrostatic attraction force and the electrostatic repulsion force are applied to the diaphragm, or symmetrically positive and negative electrostatic force is applied. On the other hand, it has the disadvantage of having difficulty in obtaining a high sound pressure, because of a low aperture ratio since sound waves are outputted through a through hole in the fixed electrode.
When the electrostatic ultrasonic transducer is used for a superdirectivity speaker, even when an ideally amplitude modulated wave in an ultrasonic band is inputted to the speaker, if the asymmetrically positive and negative distortion component of the waveform (carrier wave) outputted from the transducer is large, the distortion component becomes an audible component, so that, in addition to the ultrasonic component, the audible sound is outputted directly from the speaker, thus posing the specific problem of decreasing the directivity in audibility. This is because the electrostatic transducer has the sound-pressure characteristics of a wide frequency band (even if an audible sound itself is inputted directly, a good sound pressure can be outputted), which is a problem peculiar to a transducer with wide-band frequency characteristic. Accordingly, it is desirable to use the push-pull type with lower distortion of the output waveform than that of the pull type.
A case in which the push-pull transducer is driven by applying a sinusoidal driving signal will now be considered. When all the electrical characteristics and the mechanical shape and size of the upper electrodes and the lower electrodes of the push-pull transducer are the same, or when they have a completely vertically symmetrical structure, the diaphragm vibrates symmetrically positively and negatively (vertically). (For example, the positive amplitude and the negative amplitude are equal, as indicated by the dotted line in FIG. 13.)
However, when there are errors in shape, size, and position and electrical characteristics between the upper electrodes and the lower electrodes, the electrostatic force that acts actually on the diaphragm are different between the upper electrodes and the lower electrodes, even if the amplitudes of the driving signals supplied to the upper electrodes and the lower electrodes are equal to each other (indicated by the dotted line), as shown in FIG. 13. The diaphragm therefore does not vibrate vertically symmetrically (indicated by the solid line).
Accordingly, the super-directivity speaker using the related-art push-pull transducer, if it has an error in manufacturing, a positioning error, and variations in electrical characteristics, generates asymmetrically positive and negative distortion in the output waveform, resulting in a decrease in audible directivity.